Traffic control signals are employed for the control of both road and rail traffic. Normally, each light signal has a single light bulb as its light source, and when the bulb becomes inoperable, i.e. burns out, the signal no longer works. Frequent maintenance is required to replace the light bulb.
Traffic signals must be bright and oriented correctly, particularly for railway use. Railway signals must be oriented in precise alignment with the track direction, in order to be visible from a great distance. The light source within the signal must be properly positioned in relation to the lens system and colour filters in order to produce a signal with a uniform and properly oriented beam of light.
Attempts have been made in the past to satisfy these two objectives, viz., to produce the increased reliability achieved by having more than one light bulb in the signal, while at the same time correctly aligning the light bulb within the signal. Devices have been made providing electro-mechanical means for moving light bulbs about within the signal, so that a replacement bulb is moved into the place of a burnt bulb. However, it would be desirable to achieve these objectives without having to move light bulbs about within the signal.